In FM demodulating circuits which follow the final section of the IF amplifiers in radio communication equipment, etc, a standard practice is to obtain the modulated object signal in the form of an analog signal, even when the communication equipment itself actually needs it in the form of digial word string, and a demodulated analog signal is obtained after it is A/D (analog-to-digital) converted after appropriate processing as necessary.
On the other hand, there has been used a pulse-counting type FM demodulator which counts the FM carrier directly by use of a counter, over a given duration. Actually, in this case, conventional digital output can be obtained directly. However, the output of the IF amplifier still preserves the narrow band nature of the signal significantly, and the object signal appears merely in a small deviation over a fixed count number over each count duration. Also resolution and data rate can not be optimized; hence, the foregoing characteristics prevent this method from being widely used.
Another method is known as the period or duration measuring method; however, due to the same reasons as above stated, this method cannot be widely used.
In order to be adequate and/or suitable, for such counter type FM demodulator circuit, the IF signal to be counted must have a very wide deviation, such as, the modulation index should exceed several percents. This can be realized only by again converting the IF signal down to a still lower frequency, which only amplifies the complexity of the total circuit of the receiver.